Valdez was 1-1 with a 3.80 ERA in four starts for Reno this year with 25 strikeouts and five walks in 23 2/3 innings.
The 25-year-old got the nod over Kevin Mulvey, Billy Buckner and Bryan Augenstein.
"I think he's pitched the best," D-backs manager A.J. Hinch said. "He's had a good run of starts. He'll get his first taste of the big leagues and we'll give him the opportunity to stick."
Benson was forced to leave Wednesday's start in Denver due to a strained pectoral muscle. The team called up Mulvey on Thursday and used the starter out of the bullpen Friday against the Cubs.
Valdez was named the organization's Minor League Pitcher of the Year in 2008 when he had 13 wins between Class A Visalia and Double-A Mobile. Last season was a struggle for him at Reno, as he went 7-6 with a 4.78 ERA in 19 games [18 starts].
"He's a guy that needs to pound the strike zone and rely on his defense," Hinch said. "He lost a little bit of his aggressiveness last year and I don't know if it was because it was his first taste of Triple-A or what it was, but he lost a little of his edge and conviction. Something clicked for him in spring and it felt like the old Cesar Valdez was back."